Sesame, or Sesamum indicum, is a tropical annual cultivated for its oil and its nut flavored seeds. The sesame plant grows to a height of 2-7 feet, and at its leaf axils are found capsules which contain the sesame seed. Upon maturity in nature, the capsules holding the sesame seeds begin to dry down, the capsules normally split open, and the seeds fall out. Commercially, the harvester tries to recover as much seed as possible from mature capsules. From ancient times through the present, the opening of the capsule has been the major factor in attempting to successfully collect the seed. Harvesting methods, weather, and plant characteristics all contribute to the amount of seed recovered.
The majority of the world's sesame is harvested manually. With manual nonmechanized methods, it is desirable for the sesame seed to fall readily from the plant. Upon physiological maturity, the sesame stalks are cut, tied into small bundles, and then stacked in shocks. Further harvesting procedures vary from country to country and from area to area within countries. Some move the shocks to a threshing floor so that the seed that falls out can be recovered. Others put plastic or cloth in the fields under the shocks to catch the seed. For manual harvesting methods in which the dried, shocked sesame is moved to a threshing floor or over a plastic or cloth, preferred plant varieties include dehiscent, or super shattering, in which less than 10% of the seeds set are retained in the capsule.
Other methods involve leaving the shocks in the fields, and when the shocks are dry, the sesame is turned upside down and struck with an implement to shake out all of the seed. For this type of manual harvesting method, it is preferred that the capsule hold as much of the sesame seed as possible until the farmer inverts the stalk. Plant varieties rated as shattering which retain as much seed as possible before inversion are preferred. Common methods of manual harvest are discussed in Weiss, E. A., (1971). Castor, Sesame, and Safflower. Leonard Hill Books, London, England.
In an effort to mechanize the harvest of sesame, D. G. Langham introduced the use of swathers in Venezuela in 1944. The swathers were used to cut the sesame plants, manual labor was used to bundle and shock the cut plants, and combines were brought in to thrash the shocks. It was determined that seed shattering during mechanized harvesting methods caused considerable loss of sesame seed. While mechanization was considered to be essential for crop production in the Western hemisphere, it became obvious that the dehiscence of the sesame seed pod was the principal obstacle to the widespread acceptance of sesame as a commercial crop. (Langham, D. G. 1949. "Improvement of Sesame in Venezuela," Proceedings First International Sesame Conference, Clemson Agricultural College, Clemson, S.C., pp. 74-79). As programs to introduce sesame production in the United States in Arizona, South Carolina, Nebraska, Oklahoma, and Texas were initiated, mechanization was considered essential due to high labor costs. Kalton, one of the Texas researchers, reported that the shattering nature of available strains was the main obstacle in complete mechanization of the sesame crop. (Kalton, R. 1949. "Sesame, a promising new oilseed crop for Texas," Proc First International Sesame Conference, Clemson Agricultural College, Clemson, S.C., pp. 62-66).
In 1943, D. G. Langham found a mutation on a sesame plant. Capsules did not open on plants expressing this mutation (FIG. 8). In succeeding generations, Langham showed that it was a recessive single gene which produced this indehiscence, where all the seeds were retained inside the unopened capsule. While it was believed that indehiscence would solve the problem of seed loss on mechanized harvesting, it was found that the capsules were too tough to effectively release the seed. Many of the capsules passed through a combine without opening. When more rigorous combining was attempted, an increase in efficiency of capsule opening was achieved but at the expense of seed quality. Seeds were broken due to the more rigorous combine conditions, and the broken seeds released free fatty acids. Chemical reactions with free fatty acids led to rancidity and concomitant undesirability of the harvested seed.
The indehiscent sesame varieties described above were used by various plant breeders in an attempt to develop desirable sesame lines. In addition to traditional cross-breeding approaches, some attempted to alter the chromosome numbers through tetraploids and interspecific crosses. Yermanos attempted to improve release of seed by increasing the length of the capsule so that there would be more surface for the combine to crack the capsules open (personal communication). Unfortunately, even with a small opening on the top of the capsule, a high percentage of broken seed was found on mechanized harvesting, preventing commercial use of this sesame line.
D. G. Langham reported in the late 1950's that the placenta attachment between each sesame seed and the placenta was important in the retention of seed in the capsule. He believed that he could improve the shatter resistance of sesame with increased placenta attachment but did not believe that all the seed could be retained in the capsule. However, Yermanos reported that during capsule maturity, the placenta attachment gradually weakens and is obliterated when the capsule is completely desiccated. (Yermanos, D. M. 1980. "Sesame. Hybridization of crop plants," Am Soc Agronomy-Crop Sci of America, pp. 549-563). Thus, it appeared that the placenta attachment would have little effect on seed retention in dry, mature capsules during harvesting. A seamless gene which retained all the seed in the capsules was discovered by D. G. Langham and D. R. Langham in 1986 (FIG. 9). This was crossed with shattering types, and some progeny had an opening at the tip of the capsule. The seamless capsules were similar to the indehiscent capsules in that it was too difficult to remove the seed from the capsule without damaging the seed.
In 1982, the first non-shattering line (retaining 50-70% of the seeds set) requiring no manual labor was introduced. This line could be harvested by swathing the sesame, leaving it to dry in the field, and then picking it up by a combine. Although complete mechanization was achieved, extensive loss of seed due to adverse weather conditions continued to occur.
Other varieties were developed between 1988 and 1997 which allowed for direct combining with 70-90% seed retention, but extensive loss of seed due to wind and rain continued to occur. Lines that generally yielded 80% of the seed under ideal conditions would yield only 45-65% under adverse conditions. Thus, while many of the crosses began to moderate the deleterious effects of mechanized harvesting, none were able to increase the yields to the level of manually harvesting shattering cultivars.
New lines of sesame have now been discovered which are defined by a new category of dehiscence: non-dehiscence. These lines retain most of the seed within the capsule despite adverse weather conditions such as wind and rain. The sesame lines of the present invention retain a sufficient amount of sesame seed during mechanized harvesting to be competitive with manual harvesting. The invention permits mechanized harvesting to be used with minimization of seed breakage because extensive combining is not required to obtain practical yields. Thus, the invention permits reduction of manual labor and concomitant economical and more rapid harvesting of sesame seed as a commercial crop.